It has proven difficult to create an economically sustainable process for large-scale production of renewable liquid fuels from biomass and organic waste. Biological processes are under development but are very complex, since they require constant monitoring and strict control of the process conditions and the balance of organisms and food. Thermochemical conversion processes are also in development, but those processes are subject to catalyst degradation/deactivation over time.
Biological processes for the production of alcohol based liquid fuels (methanol, ethanol, propanol and butanol) from lignocellulosic biomass (as opposed to grain based processes) are numerous but are not commercially viable on a large scale. Biological processes for the generation of drop-in liquid fuels (gasoline, jet and diesel fuels) from biomass usually require a designer/genetically modified microorganism as the basis of the technology, which may not be reliable at industrial scale. Of the conventional grain based biological ethanol fuel processes in operation, most generate biogenic carbon dioxide (CO2) gas as a byproduct. Although this carbon dioxide gas is biogenic and non-fossil, it nevertheless adds to global CO2 emissions. Thus, a non-fossil energy conversion method to recycle the CO2 into a renewable liquid fuel, which would have a positive effect on global greenhouse gas emissions, would be desirable.
Anaerobic digestion is a type of biochemical process in which chemical reactions carried out by various microorganisms, so called biochemical reactions, are used to decompose organic matter in the absence of oxygen. The process of anaerobic digestion is normally used for waste water treatment systems and other watery organic material treatment systems, as long as the solids can be introduced to the system at an acceptable concentration for sustaining the anaerobic digestion. That concentration is less than 2%. The products of biochemical digestion are biogases, which can be used as a clean and renewable form of energy which can be a substitute for conventional sources of energy which may be causing ecological and/or environmental problems and at the same time are depleting at a faster rate.
Various types of pretreatment technologies have been explored to enhance the rate-limiting hydrolysis step in anaerobic digestion, including mechanical, thermal, chemical, and biological pretreatment for liquefaction of certain biomass components. However, the compositional variations between biomass types cause changes in composition and yield of the products of liquefaction since lignin, hemicelluloses, and cellulose react differently during pretreatment.
Woody biomass materials that are by-products from activities such as forest harvesting, products manufacturing, construction, and demolition debris harvesting or management, are referred to as “wood residues”. Wood residues can be inexpensive sources of biomass, and they are the most common biomass fuel for heat or power generation. It has been suggested that in the future, fast growing grasses, shrubs and tree hybrids (i.e. energy crops such as miscanthus and switchgrass) could be grown for use in the production of fuels or other products.
Known anaerobic digestion (AD) processes have limited utility for high-solids woody feedstocks and are subject to overgrowth of hydrogentrophic methanogens and acetogens, leading to reduced hydrogen yields and the production of unwanted by-products. Thus, significant dilution and careful adjustment of the feedstock solids to less than 2% solids content is normally required. Moreover, a capital intense reforming process is needed for conversion of the mixed biogas stream produced to syngas, especially for removal of any excess CO2, which has a highly deleterious effect on a gas-to-liquid fuel (GTLF) process.
The Fischer Tropsch (FT) reaction is a known GTLF process for use in converting a syngas containing H2 and CO into synthetic hydrocarbons. Several processes for the generation of FT syngas are known. Thermochemical processes for the conversion of biomass or organic wastes into syngas by thermally gasifying the feedstock are known and can be used to produce both alcohols and drop-in liquid fuels. However, such syngases have a high contaminant load, for example particulates such as char and condensable vapours such as those derived from fast pyrolysis and can also contain non-reactive gases.
The FT process requires the use of a catalyst. Most gas reaction catalysts are highly sensitive to particulate and/or vaporous contaminants entrained in the syngas and become quickly fouled by those contaminants. Any syngas containing such contaminants, must therefore be cleaned, often at significant cost, to achieve the high quality (high purity/cleanness) syngas required for reliable operation of the Fischer Tropsch (FT) reaction. Other contaminants in the syngas, such as non-reactive gases, for example N2, CO2, CH4, SO2 can also be detrimental to the FT process and, depending on the gas, can degrade the catalyst, decrease the catalyst production rate, or affect the desired product produced in the reactor by changing the H2/CO ratio entering the reactor. Absent any economical source of clean syngas with the proper H2/CO ratio, these problems render synthetic fuel production with Fischer Tropsch from thermochemically gasified biomass uneconomical and impractical.
The syngas required for the FT reaction can also be created commercially by reforming methane. A synthetic methane production process is known which uses atmospheric CO2 and H2 generated by water electrolysis. This process can be used to create renewable/non-fossil methane if renewable or nuclear electricity is used. However, high electricity costs render this process uneconomical on a large continuous scale, unless excess renewable electricity is available. Yet, the supply of excess renewable electricity is erratic and often unpredictable, making its use in a standalone process economically difficult, if not impractical, due to large swings in the amount of energy available.
In a first methane reforming process, the “wet” reforming process (SRM), water is reacted with methane in a catalytic reformer reactor, to carry out the following basic reaction:CH4+H2O=3H2+CO.This results in a 3 to 1 molar ratio of H2 and CO in the syngas.
In a second methane reforming process, the “dry” (DRM) reforming process, CO2 is reacted with CH4 in a catalytic process according to the basic reactionCO2+CH4=2H2+2CO.
This results in a 1 to 1 molar ratio of H2 and CO in the syngas. The more CO2 is used to replace H2O in the methane reformer, the more “dry” the process will be, i.e. the less water will be used and the molar ratio between H2 and CO will vary from a low of 1 (completely dry reaction) to a high of 3 (completely wet reaction). The overall reactions which take place at the same time in reforming are as follows:CH4+H2OCO+3H2 ΔH298θ=+206 kJ·mol−1 CH4+2H2OCO2+4H2 ΔH298θ=+165 kJ·mol−1 CO+H2OCO2+H2 ΔH298θ=−41 kj·mol−1 CH4+CO22H2+2COΔH298θ=+247 kJ·mol−1 
Although dry (CO2) reforming of methane (DRM) is a well-studied reaction that has both scientific and industrial importance, significant technical hurdles still exist with DRM utilization. The DRM reaction requires high temperatures (about 900 C) and is highly endothermic (20% more than the pure SRM reaction), thereby requiring significant amounts of high grade energy for total reactant conversion. In addition, severe catalyst degradation occurs due to carbon deposition. Since molecular carbon formation is a common problem of the known DRM process, significant amounts of water are always used resulting in a process which is mostly wet, i.e. it is a slightly modified SRM process, not a mostly dry or DRM process. This means that water supplies most of the oxygen for the CO in the syngas. That makes existing processes environmentally costly as it is environmentally beneficial to split a CO2 molecule which would otherwise enter the atmosphere adding to the greenhouse effect rather than splitting H2O, but doing so means 20% more reforming energy will need to be used.
Thus, an improved process for the production of liquid hydrocarbon fuels from non-fossil sources would be desirable.